EP-A2 No. 0209857, corresponding to U.S. Ser. No. 887,934 filed July 22, 1986, now U.S. Pat. No. 4,711,934, discloses cationic paint binders having the schematic structure ##STR1## wherein E--is the radical of an epoxy resin,
PH--is the radical of a phenol, PA1 A--is the radical of an amine and PA1 DI--NCO is the radical of a semi-blocked diisocyanate.
These binders can be produced in a surprisingly uncomplicated manner and, upon protonation, give water-dilutable paints with good applicational properties and fully crosslinked films at a stoving temperature as low as about 160.degree. C. However, it has been determined that coatings produced with the binders do not meet certain stringent requirements, particularly those required by the automobile industry with regard to various corrosion tests such as salt spray resistance and the so-called scab-test.
It has now been found that condensates of the type described in the above patent, prior to the reaction with the epoxy resin, can be linked in a controllable manner through basic NH-functional compounds and formaldehyde. Because of this controlled linking, the corrosion protection of the paints formulated therewith can be considerably enhanced, in particular if the linking is effected through NH-functional epoxy resin amine adducts.